In mathematics Paolo introduced the period or comma as a device for separating numbers into groups of three for easing calculations on the order of thousands and millions. He is most famous for his work on equations (aequationibus) that fused geometry and arithmetic, which we today would recognise as algebra. His most important mathematical treatise was the Regoluzze, a manual of elementary arithmetic, written in 1340. Some of the "little rules" are:[5]

1. If you wish to write down [a number of] many figures, make a period at every third figure beginning from the right hand and going towards the left, and then you will have as many thousands as are in front of the periods.
2. If you wish to multiply numbers ending with a zero, multiply their figures and put all of the zeros at the end.
15. If you wish to multiply fraction by fraction, multiply the numerators with one another, and the denominators similarly.
32. If you multiply the width of a circle by 22 and divide by 7, you will have the circumference.